Protocol - Body Mass Index
Standing height is measured from the top of the participant’s head to his or her heels. Measured weight is assessed using a digital floor scale that should be calibrated daily using standardized weights, and a log of calibration results should be maintained. These measurements are taken among participants who are able to stand unassisted.
Body mass index (BMI) is derived from the Standing Height protocol (under the Height measure) and the Measured Weight protocol (under the Weight measure), both from the Anthropometrics domain. Self-reported values of height and weight are considered to be less accurate than those obtained from measurement protocols, so the PhenX Anthropometrics Working Group strongly recommends supervised measurement of height and weight using instrument protocols.
Under usual field conditions, for reliability, the Anthropometrics Working Group suggests that the measurements are taken in duplicate. A third height measurement should be taken if the first two measurements differed by >1.0 cm (1/4"). A third weight measurement should be taken if the first two measurements differ by >200 grams or 0.25 pounds. In either case, if it is necessary to take a third measurement, the two closest measurements are averaged. Should the third measurement fall equally between the first two measurements, all three should be averaged.
Additionally, when measuring weight, the observer should determine whether the woman is pregnant and, if she is pregnant, request how many weeks pregnant. Pregnancy status may be sensitive information, so participants should be asked this question in a private setting.
There are several overarching, critical issues for high-quality data collection of anthropometric measures. These issues include (1) the need for training (and retraining) of study staff in anthropometric data collection; (2) duplicate collection of measurements, especially under field conditions; (3) use of more than one person for proper collection of measurements, where required; (4) accurate recording of the protocols and the measurement units of data collection; and (5) use of required and properly calibrated equipment.
Note: Detailed videos illustrating the measurement procedures can be found on the National Health and Nutrition Examination Survey (NHANES) website.
Standing Height Protocol Ask the participant to remove hair ornaments, jewelry, buns, or braids from the top of the head that interfere with the measurement. Shoes should be removed.
Ask the participant to stand erect against the backboard with the body weight evenly distributed and both feet flat on the stadiometer platform (Exhibit 1). The participant’s feet should be positioned with the heels together and toes pointed slightly outward at approximately a 60 degree angle. Check to be sure that the back of the head, shoulder blades, buttocks, and heels make contact with the backboard of the stadiometer.
Note: Depending on the overall body conformation of the individual, all four contact points-head, shoulders, buttocks, and heels-may or may not touch the stadiometer backboard (Exhibit 2). For example, elderly survey participants may have kyphosis, a forward curvature of the spine that appears as a hump at the upper back. In particular, dowager’s hump is a form of kyphosis that creates a hump at the back of the neck. Additionally, some overweight survey participants cannot stand straight while touching all four contact points to the backboard. In such instances, it is important to obtain the best measurement possible according to the protocol.
Stature measurements are made with the head aligned in the Frankfort horizontal plane (Exhibit 2). The head is in the Frankfort plane when the horizontal line from the ear canal to the lower border of the orbit of the eye is parallel to the floor and perpendicular to the vertical backboard (see Exhibit 2). Many people will assume this position naturally, but for some survey participants the examiner may need to gently tilt the head up or down to achieve the proper alignment. Instruct the survey participant to look straight ahead.
If you cannot position the participant such that his or her trunk remains vertical above the waist, such that the arms and shoulders are relaxed, and such that the head is positioned in the Frankfort plane, be sure to note this in the measurement record. This information might be useful to interpret study findings. In the National Health and Nutrition Examination Study 2007-2008, a comment described as "Not Straight" is noted in the stature record.
Once positioned, lower the stadiometer headpiece so that it rests firmly on top of the participant’s head, with sufficient pressure to compress the hair. Instruct the survey participant to stand as tall as possible, take a deep breath, and hold this position. The act of taking a deep breath helps straighten the spine to yield a more consistent and reproducible stature measurement. Notice that the inhalation will cause the headpiece to rise slightly.
As soon as the participant inhales, record the measurement. After recording the measurement, tell the participant to relax. Once the measurement is taken, raise the stadiometer headpiece and have the participant step away from the stadiometer.
Adjustments for shoes and hair: When participants cannot remove hair braids, buns, and headwear that interfere with the stature measurement, measure the distance from the scalp to the top of the hair with a small ruler to the nearest 0.1 cm. If shoes are worn, measure the height of the shoe heel to the nearest 0.1 cm. A corrected height value can be calculated by subtracting these distances from the original stature measurement, thus yielding an adjusted stature value.
Record standing height in meters (or centimeters) or inches. Repeat height measurement.
Current Measured Weight Protocol
Note: Detailed videos illustrating the procedure can be found on the NHANES website.
A digital scale or beam balance is used to weigh participants.
Participants are asked to wear an examination gown and socks or light clothes without shoes. Only undergarments are worn beneath the gown. The procedures for obtaining the weight measurement are as follows:
The examiner briefly informs the participant that his/her weight will be measured. Participants are asked to remove objects such as cell phones, wallets, and toys from their pockets.
- The health technician directs participants to stand in the center of the scale platform with hands at their sides and looking straight ahead.
- The weight measurement is recorded in kilograms or pounds.
- Special situations:
- Small children: Infants and toddlers who cannot stand alone on the scale will be weighed with an adult, with an infant’s scale. If an adult is holding the child, then the adult guardian or the health technician will stand alone on the scale so the scale can be tared. This sets the scale readout to zero. The child is then handed to the adult and the child’s weight is measured.
- If the participant is wearing a cast or medical prosthesis, make a note in the record of the location and place.
- If the participant wore street clothes instead of the examination gown, make a note of this in the record. It is acceptable for infants to wear diapers or underpants and a t-shirt.
- Participants should not be weighed if they are wearing shoes.
- Note that special consideration may be needed for participants whose weight exceeds the capacity of the study scale. For example, weight can be obtained using two portable scales:
- Have the participant stand with one foot on each portable scale.
- Combine the two results to approximate the weight.
- Record the weight.
- If the weight equals the capacity of both portable scales, note that the weight Equals Capacity (EC) of the scales.
Record current weight in pounds or kilograms. Repeat weight measurement.
Derivation of Body Mass Index (BMI)
BMI calculations are recommended in children (2+ years) and adults, but some references suggest that the calculations are relevant from birth.
Formula and Calculation
Kilograms and meters (or centimeters)
Formula: BMI = weight (kg) / [height (m)]2
Pounds and inches
Formula: BMI = weight (lb) / [height (in)]2 x 703
Source: Centers for Disease Control and Prevention website. Body Mass Index page.
Interpretation of BMI in Adults
The following categories are used to interpret BMI in both adult men and women (20 years old and older).
- Underweight : below 18.5
- Normal weight: 18.5-24.9
- Overweight: 25-29.9
- Obesity: 30 and above
Source: Centers for Disease Control and Prevention website. Body Mass Index page.
Interpretation of BMI in Children and Teens
BMI for children and teens is age- and sex-specific, so it is often referred to as BMI-for-age. After calculating BMI, the BMI number should be plotted on the BMI-for-age growth chart (available for boys and girls, separately, from the Centers for Disease Control and Prevention website) to obtain a percentile ranking. The following categories are used to interpret BMI-for-age in children and teens.
- Underweight: less than the 5th percentile
- Normal weight: 5th percentile to less than the 85th percentile
- Overweight: 85th to less than the 95th percentile
- Obesity: equal to or greater than the 95th percentile
Personnel and Training Required
Technicians should be trained in the basic techniques of anthropometric measurements.
Stadiometer with a fixed-length backboard and an adjustable headpiece.
Digital scale or beam balances. Portable scales have also been used in the National Health and Nutrition Examination Survey. Standard weight to calibrate scale. A tare function is used when weighing infants and children. The tare function is a feature found in clinical scale equipment.
|Specialized requirements for biospecimen collection||No|
|Average time of greater than 15 minutes in an unaffected individual||No|
Mode of Administration
Secondary Data Analysis
Toddler, Child, Adolescent, Adult, Senior
The standing height protocol is used for participants 2 years old or older who can stand unassisted. The current measured weight protocol is used for all ages. Therefore, body mass index (BMI) calculations are recommended in children (2+ years) and adults. Adult participants who cannot stand unassisted are excluded.
The National Health and Nutrition Examination Survey (NHANES) 2007-2008 protocols were selected as best-practice methodology and are the most widely used protocols to assess measured height and weight. Both protocols are used to calculate body mass index, assess nutritional status, and predict morbidity and mortality.
English, Other languages available at source
|caDSR Common Data Elements (CDE)||Person Body Mass Index Value||2006410||CDE Browser|
|Human Phenotype Ontology||Abnormality of body mass index||HP:0045081||HPO|
|Human Phenotype Ontology||Increased body mass index||HP:0031418||HPO|
|Human Phenotype Ontology||Decreased body mass index||HP:0045082||HPO|
Process and Review
This section will be completed when reviewed by an Expert Review Panel.
Protocol Name from Source
National Health and Nutrition Examination Survey (NHANES), Anthropometrics procedure manual, 2007-2008
Centers for Disease Control and Prevention (CDC), National Center for Health Statistics (NCHS). (2007-2008). National Health and Nutrition Examination Survey anthropometrics procedure manual. Hyattsville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention.
|Variable Name||Variable ID||Variable Description||dbGaP Mapping|
|PX570101080100||calculated BMI in kg/m2||Variable Mapping|
|PX570101080200||calculated BMI in lb/in2||N/A|
|PX570101050100||Record whether the participant is wearing a more||N/A|
|PX570101050200||If the participant is wearing one, record more||N/A|
|PX570101070500||Measured weight in kg, first measurement||N/A|
|PX570101070100||Measured weight in lbs, first measurement||N/A|
|PX570101070600||Measured weight in kg, second measurement||N/A|
|PX570101070200||Measured weight in lbs, second measurement||N/A|
|PX570101070700||Measured weight in kg, third measurement||N/A|
|PX570101070300||Measured weight in lbs, third measurement||N/A|
|PX570101070800||Measured weight in kg, average||N/A|
|PX570101070400||Measured weight in lbs, average||N/A|
|PX570101010100||Standing Height measured in centimeter, more||N/A|
|PX570101010200||Standing Height measured in centimeter, more||N/A|
|PX570101010300||Standing Height measured in centimeter, more||N/A|
|PX570101020100||Standing Height measured in centimeter, more||N/A|
|PX570101020200||Standing Height measured in centimeter, more||N/A|
|PX570101020300||Standing Height measured in centimeter, more||N/A|
|PX570101030100||Standing Height measured in centimeter, more||N/A|
|PX570101030200||Standing Height measured in centimeter, more||N/A|
|PX570101030300||Standing Height measured in centimeter, more||N/A|
|PX570101040100||Standing Height measured in centimeter, average||N/A|
|PX570101040200||Standing Height measured in centimeter, average||N/A|
|PX570101040300||Standing Height measured in centimeter, average||N/A|
|PX570101060000||Record whether or not the participant is more||N/A|
Body Mass Index
February 24, 2012
Body mass index (BMI) is derived from height and weight measurements. Height is the distance from the top of the participant’s head to the heels of his or her feet (i.e., the vertical length). Current measured weight is the weight in kilograms or pounds of the participant. BMI is calculated through a formula in which weight is divided by height squared.
Body mass index (BMI) is a measure of body fat, based on height and weight. Height or stature is used to assess body size and bone length. Current weight is used to assess participant’s growth, development, and current health status. Higher BMI levels are associated with various health-related co-morbidities.
body mass index, Anthropometrics, Anthropometrics Procedure Manual, Anthropometry Manual, arthritis, Beam Balance, BMI, body mass index, Body Size, cardiovascular disease, Calipers, CDC, Centers for Disease Control and Prevention, diabetes, Digital Floor Scale, Frankfort Horizontal Plane, height, National Health and Nutrition Examination Survey, NHANES, obesity, ponderal index, Standing Height, stature, Stadiometer, weight, SAA, substance abuse and addiction
|Protocol ID||Protocol Name|
|570101||Body Mass Index|